1. Field of the Invention
The present invention relates to methods of manufacturing micromachinable, pico- to nanoliter-volume, hermetic packaging that incorporates reliable electrical feedthroughs, and sensors configured utilizing the same, all of which are intended to perform reliably in harsh and biological environments.
2. Description of the Prior Art
Over the past 20 years, advances in the field of microelectronics have enabled the realization of microelectromechanical systems (MEMS) and corresponding batch fabrication techniques. These developments have allowed the creation of sensors and actuators with micrometer-scale features. With the advent of the above-described capability, heretofore implausible applications for sensors and actuators are now significantly closer to commercial realization.
In parallel, much work has been done in the development of pressure sensors. Pressure sensors are disclosed, for example, in U.S. Pat. No. 6,855,115, issued Feb. 15, 2005; U.S. patent application Ser. No. 10/054,671, filed Jan. 22, 2002; U.S. patent application Ser. No. 10/215,377, filed Aug. 7, 2002; U.S. patent application Ser. No. 10/215,379, filed Aug. 7, 2002; U.S. patent application Ser. No. 10/943,772, filed Sep. 16, 2004; and U.S. patent application Ser. No. 11/157,375, filed Jun. 21, 2005, all of which are incorporated herein by reference.
In particular, absolute pressure sensors, in which the pressure external to the sensor is read with respect to an internal pressure reference, are of interest. The internal pressure reference is a volume within the sensor, sealed, which typically contains a number of moles of gas (the number can also be zero, i.e. the pressure reference can be a vacuum, which can be of interest to reduce temperature sensitivity of the pressure reference as known in the art). The external pressure is then read relative to this constant and known internal pressure reference, resulting in measurement of the external absolute pressure. For stability of the pressure reference and assuming the temperature and volume of the reference are invariant or substantially invariant, it is desirable that the number of moles of fluid inside the reference does not change. One method to approach this condition is for the reference volume to be hermetic.
The term hermetic is generally defined as meaning “being airtight or impervious to air.” In reality, however, all materials are, to a greater or lesser extent, permeable, and hence specifications must define acceptable levels of hermeticity. An acceptable level of hermeticity is therefore a rate of fluid ingress or egress that changes the pressure in the internal reference volume (a.k.a. pressure chamber) by an amount preferably less than 10 percent of the external pressure being sensed, more preferably less than 5 percent, and most preferably less than 1 percent over the accumulated time over which the measurements will be taken. In many biological applications, an acceptable pressure change in the pressure chamber is on the order of 1.5 mm Hg/year.
The pressure reference is typically interfaced with a sensing means that can sense deflections of boundaries of the pressure reference when the pressure external to the reference changes. A typical example would be bounding at least one side of the pressure reference with a deflectable diaphragm or plate and measuring the deflection of the diaphragm or plate by use of, among other techniques, a piezoresistive or a capacitance measurement. If the deflection of the diaphragm or plate is sufficiently small, the volume change of the pressure reference does not substantially offset the pressure in the pressure reference.
These approaches may require an electrical feedthrough to the hermetic environment (e.g., to contact electrodes inside the hermetic pressure reference), for connection to outside electronics to buffer or transmit the signal. Alternatively, electronics may be incorporated within the reference cavity, requiring power to be conducted into the hermetic environment. To maintain stability of the pressure reference, these seals should also be hermetic, resulting in the necessity to develop a feedthrough technology for contacts through the cavity walls. As is known in the art, such feedthrough points are typically sites for failure of hermeticity. This problem is further exacerbated when miniaturizing the sensor, since the total volume of material available for hermetic sealing shrinks proportionally and the reliability of the feedthrough is also greatly reduced. In the limit of ultraminiaturized sensors, such as those producible using microelectromechanical systems (MEMS) technology, one of the major challenges to enabling the use of such devices in applications where they are physically connected to other devices has been the creation of reliable hermetic packaging that provides feedthroughs that enable exchange of power and information with external electronics.
Design criteria for ultraminiature packaging that overcomes the aforementioned shortcomings are as follows: The packaging must exhibit long term hermeticity (on the order of the life of the sensor, which in some cases can exceed tens of years). Feedthroughs must be provided through the hermetic package that do not introduce new or unnecessary potential modes of failure. The feedthroughs will constitute a necessary material interface, but all other interfaces can and should be eliminated. In other words, the number and area of material interfaces should be minimized to reduce the potential for breach of hermeticity. The materials selected must be compatible with the processes used to fabricate the package as well as sufficiently robust to resist deleterious corrosion and biocompatible to minimize the body's immune response. Finally, the packaging should be amenable to batch fabrication.
In the past, many methods for creating such hermetic packages have been proposed. One approach used in the past to create the pressure cavity is anodic bonding to create a silicon-to-glass seal. A borosilicate glass is required for this method. Another technique utilized in the creation of hermetic packages is eutectic bonding to create a silicon to metal hermetic seal, e.g. Au to Si. Both of these bonding methods used to create the pressure cavity introduce a large area along the perimeter of the material interface of the pressure cavity package which presents opportunity for failure, e.g. through corrosion. These methods for creating the pressure cavity do not minimize the area of the material interface as is desirable. A desirable improvement to the construction of the pressure cavity would minimize the material interface to the hermetic electrical feedthroughs, and, even further, minimize the number and area of material interfaces in those feedthroughs.
Previous attempts to create hermetic feedthroughs also fall short of the above-stated requirements. Many prior art hermetic feedthroughs are too large and not amenable to the required miniaturization for pico to nanoliter volume packaging achievable by MEMS or similar approaches. Furthermore, earlier attempts to create feedthroughs in pico to nanoliter packaging are prone to corrosion because of the materials used in construction or are sufficiently complicated that they introduce more material interfaces than are necessary. A representative feedthrough approach, known as a “buried” feedthrough, is illustrated in FIGS. 1-5. One method for creating a buried feedthrough is as follows: a metal 10 is deposited onto substrate 12 in a predefined pattern, as shown in FIG. 1. An insulating layer 14 is deposited on top of the metal layer, as shown in FIG. 2, and this insulating layer 14 is polished to planarize this surface. In FIG. 3 an etchant has been used to expose the metal layer at input and output sites 16, 18 for the feedthroughs. In FIG. 4, another substrate 20 is bonded on top of this structure, forming a hermetic cavity 22. A eutectic bonding method is illustrated, which involves the use of gold deposits 24 interposed between the insulating layer 14 and the upper substrate 20 to bond the upper substrate to the insulating layer. In FIG. 5 the upper substrate 20 is machined to expose the external feedthrough 18. An electrical component can now be connected to the external feedthrough 18, whereupon electrical communication is established through metal 10 to the internal feedthrough 16 and the interior of the hermetically sealed chamber 22
This prior art buried feedthrough suffers a number of disadvantages. First, there are numerous material interfaces: an interface 30 between the lower substrate 12 and the metal 10; an interface 32 between the metal 12 and the insulating layer 14, an interface 34 between the insulating layer 14 and the gold 24; and an interface 36 between the gold 24 and the upper substrate 20, all of which create potential paths for infusion into or effusion out of the hermetic chamber 22. The creation of this buried feedthrough also introduces increased processing steps. Further, the insulating layer material is cited as being prone to corrosion in certain environments, e.g. the human body. Corrosion issues may be further exacerbated by the application of electrical bias to metal 10 which may be required in certain applications. Thus prior art hermetic feedthroughs fall short of meeting the constraints outlined above.
Also, many prior art attempts to provide pressure sensors utilize silicon as a substrate material. If the package is implanted in vivo, silicon is not an optimal material choice. Silicon invokes an undesirable immune response over other, more inert materials such as fused silica. If silicon is used, a coating must be applied to ensure biocompatibility. Such a coating increases the package size, thereby decreasing the benefits of miniaturization, and introduces an undesirable additional processing step in the manufacture of the package.
Additionally, prior art devices commonly employ the use of borosilicate glass as part of the pressure cavity. The ions in borosilicate glass constitute an impurity in the glass. The barrier to diffusion of water decreases as the purity of glass decreases. This makes use of impure glass undesirable in such applications.
Thus a need exists for hermetic pico to nanoliter packaging with electrical feedthroughs for use in biological environments, such packaging being constructed of high-purity materials and having a reduced number and area of material interfaces.